HIGH-ENERGY GAMMA-RAY EMISSION FROM PION DECAY IN A SOLAR-FLARE MAGNETIC LOOP

We have investigated the production of high-energy gamma rays resulting from pion decay in a solar flare magnetic loop. We took into account magnetic mirroring, MHD pitch-angle scattering, and all of the relevant loss processes and photon production mechanisms. We treated the transport of both the primary ions and the secondary positrons resulting from the decay of the positive pions, as well as the transport of the produced gamma-ray emission. We calculated the distributions of the gamma rays as a function of atmospheric depth, time, emission angle, and photon energy and studied the dependence of these distributions on the model parameters. The obtained angular distributions are not sufficiently anisotropic to account for the observed limb brightening of the greater than 10 MeV flare emission, indicating that the bulk of this emission is bremsstrahlung from primary electrons. We compared our calculations with the available data for pion decay radiation from the 1982 June 3 flare and considered the possible models of particle transport and gamma-ray production for this flare.